This invention relates in general to the formation of coatings by plasma spraying techniques, and more particularly, to the production of magnetic-cermet dielectric coatings by plasma spraying of composite particles.
In the past, magnetic-cermet dielectric coatings have been formed by coating a mixture of metal particles and ceramic particles in a liquid carrier onto a substrate, evaporating the liquid and heating the assembly in air to the sintering temperature of the ceramic for an hour or more. A dielectric layer results, with the metal particles sufficiently separated (by spacing or by in-situ formation of insulating oxide layers) so that the coating has sufficiently low electrical conductivity.
While suitable for some applications, this process has a number of disadvantages and problems. Parts to be coated are limited in size to the sizes of sintering furnaces available. Since the sintering temperature is quite high for a relatively long period (often over 800.degree. C.), depending on the ceramic or glass frit employed, substrates such as polymeric materials or carbon--carbon composites that would be damaged at the sintering temperature in air cannot be used. Also, cooling from the sintering temperature may, where the coating and substrate have significant differences in coefficient of thermal expansion (CTE), cause cracking or delamination in the coating. Repair of damaged areas is difficult where the entire part must be exposed to the sintering conditions.
The ability to coat a variety of substrates of various sizes would expand the usefulness of magnetic-cermet dielectric coatings beyond the present microwave absorption uses. For example, microwave absorbing coatings on cookware and browning dishes for use in microwave ovens would benefit from the capability of induction heating at or near food surfaces. These coatings would have applications in induction heating coatings for industrial processes such as heat treatment of non-ferrous alloys. They also may find application in interference fit assembling, welding, brazing, etc. and in the production of magnetic coatings on non-ferrous alloy plates for computer memory disk drives. Because of their high temperature resistance, they will have applications as a microwave absorber for aircraft engine exhaust components and the like.
Thus, there is a continuing need for improved methods of forming magnetic-cermet dielectric coatings which overcome the above noted problems.